The present invention is directed to a process for producing sodium bicarbonate and boric acid by carboammoniac attack of borosodium ores, or minerals, such as tincal, or borax.
In the processes for treatment of borate minerals, borocalcium minerals, for example colemanite (2CaO.3B.sub.2 O.sub.3.5H.sub.2 O), are generally transformed into boric acid while the borosodium minerals such as tincal (borax) (Na.sub.2 O.2B.sub.2 O.sub.3.10H.sub.2 O) and the borocalcosodium minerals such as boronatrocalcite (ulexite) (Na.sub.2 O.2CaO.5B.sub.2 O.sub.3.16H.sub.2 O) are most often converted into borax or sodium borate in order to valorize their Na.sub.2 O content (Brauer u. d' Ans - Fortschritte in der anorganischen Chemie - 1877-1917 -- (1922) -- Vol. I -- Part 2, page 2596).
Thus, in French Patent No. 1,256,827 of May 11th, 1960, filed by S.p.A. Larderello, there is described a process for producing boric acid by carboammoniac attack of colemanite or other borocalcium minerals. According to this patent, one begins, for example, with colemanite, which is ground, possibly calcined, then treated with a solution of ammonium bicarbonate and ammonia or carbonic anhydride and ammonia, while being agitated, for several hours, at a temperature between ambient temperature and 90.degree. C. to produce the reaction: EQU 2CaO.3B.sub.2 O.sub.3.5H.sub.2 O+ 2 (NH.sub.4)HCO.sub.3 + 4NH.sub.3 + 2H.sub.2 O.fwdarw.2CaCO.sub.3 + 6(NH.sub.4)H.sub.2 BO.sub.3
the resulting suspension being filtered in order to separate the calcium carbonate, and the remaining solution of ammonium borates being brought to the boiling point in order to concentrate it and to eliminate the ammonia which is recycled: EQU (NH.sub.4) H.sub.2 BO.sub.3 .fwdarw.H.sub.3 BO.sub.3 + NH.sub.3.
After a substantial concentration of the solution, it is cooled in order to crystallize the boric acid; the crystals are separated, washed and dried while the mother liquors return to the concentration circuit.
The borosodium minerals such as tincal (borax) and kernite (Na.sub.2 O.2B.sub.2 O.sub.3.4H.sub.2 O) are normally treated by simple recrystallization to produce pure borax. If it is desired to finally obtain boric acid, the mineral or the pure borax is treated with an acid, for example sulphuric acid (Ullmann's Encyklopaedie der technischen Chemie -- 3rd edition -- 1953 -- Vol. 4 -- pages 587 and 591-593).
Applicant has discovered that it is worthwhile to apply to borosodium minerals not containing calcium, particularly to tincal, the carboammoniac attack proposed by Larderello S.p.A. for borocalcium minerals. Instead of precipitating calcium carbonate, one obtains sodium bicarbonate which can be separated and converted into sodium carbonate by calcination, while yielding at the same time a rich carbonic anhydride usable for the carboammoniac attack.